1. Technical Field
The present disclosure relates generally to a Universal Mobile Telecommunication System (UMTS), and more particularly, to a method and system for assigning an optimal Orthogonal Variable Spreading Factor (OVSF) code, also known as a channelization code, for separation of downlink channels to a new channel, while maintaining the existing OVSF codes.
2. Background of the Related Art
Spreading is applied to the physical channels of a UMTS, an asynchronous IMT-2000 system. Spreading consists of two operations. The first operation is the channelization operation, which transforms every data symbol into a number of chips, thus increasing the bandwidth of the signal. The number of chips per data symbol is called the Spreading Factor (SF). The second operation is the scrambling operation, where a scrambling code is applied to the spread signal.
During the channelization operation, data symbols on so-called I- and Q-branches are independently multiplied with an OVSF code. With the scrambling operation, the resultant signals on the I- and Q-branches are further multiplied by a complex-valued scrambling code, where I and Q denote real and imaginary parts, respectively. The channelization codes are OVSF codes that preserve the orthogonality between a user's different physical channels. The OVSF codes can be defined using the code tree of FIG. 1.
In FIG. 1, the channelization codes are uniquely described as Cch,SF,k, where SF is the spreading factor of the code and k is the code number, 0≦k≦SF−1. Each level in the code tree defines channelization codes of length SF, corresponding to a spreading factor of SF in FIG. 1.
The generation method for the channelization code is defined as:Cch,1,0=1,      [                                        C                                          c                ⁢                                                                  ⁢                h                            ,              2              ,              0                                                                        C                                          c                ⁢                                                                  ⁢                h                            ,              2              ,              1                                            ]    =            [                                                  C                                                c                  ⁢                                                                          ⁢                  h                                ,                1                ,                0                                                                        C                                                c                  ⁢                                                                          ⁢                  h                                ,                1                ,                0                                                                                        C                                                c                  ⁢                                                                          ⁢                  h                                ,                1                ,                0                                                                        -                              C                                                      c                    ⁢                                                                                  ⁢                    h                                    ,                  1                  ,                  0                                                                        ]        =                            [                                                    1                                            1                                                                    1                                                              -                  1                                                              ]                ⁢                                  [                                                            C                                  c                  ⁢                                                                          ⁢                  h                  ⁢                                                                          ⁢                                      2                                          (                                              n                        +                        1                                            )                                                        ⁢                  0                                                                                                        C                                  c                  ⁢                                                                          ⁢                  h                  ⁢                                                                          ⁢                                      2                                          (                                              n                        +                        1                                            )                                                        ⁢                  1                                                                                                        C                                  c                  ⁢                                                                          ⁢                  h                  ⁢                                                                          ⁢                                      2                                          (                                              n                        +                        1                                            )                                                        ⁢                  2                                                                                                        C                                  c                  ⁢                                                                          ⁢                  h                  ⁢                                                                          ⁢                                      2                                          (                                              n                        +                        1                                            )                                                        ⁢                  3                                                                                        ⋮                                                                          C                                                      c                    ⁢                                                                                  ⁢                    h                    ⁢                                                                                  ⁢                                          2                                              (                                                  n                          +                          1                                                )                                                              ⁢                                          2                                              (                                                  n                          +                          1                                                )                                                                              -                  2                                                                                                        C                                                      c                    ⁢                                                                                  ⁢                    h                    ⁢                                          .2                                              (                                                  n                          +                          1                                                )                                                              ⁢                                          2                                              (                                                  n                          +                          1                                                )                                                                              -                  1                                                                    ]            =              [                                                            C                                  c                  ⁢                                                                          ⁢                  h                  ⁢                                                                          ⁢                                      2                    n                                    ⁢                                                                          ⁢                  0                                                                                    C                                  c                  ⁢                                                                          ⁢                  h                  ⁢                                                                          ⁢                                      .2                    n                                    ⁢                                                                          ⁢                  0                                                                                                        C                                  c                  ⁢                                                                          ⁢                  h                  ⁢                                                                          ⁢                                      2                    n                                    ⁢                                                                          ⁢                  0                                                                                    -                                  C                                      c                    ⁢                                                                                  ⁢                    h                    ⁢                                                                                  ⁢                                          2                      n                                        ⁢                                                                                  ⁢                    0                                                                                                                          C                                  c                  ⁢                                                                          ⁢                  h                  ⁢                                                                          ⁢                                      2                    n                                    ⁢                                                                          ⁢                  1                                                                                    C                                  c                  ⁢                                                                          ⁢                  h                  ⁢                                                                          ⁢                                      2                    n                                    ⁢                                                                          ⁢                  1                                                                                                        C                                  c                  ⁢                                                                          ⁢                  h                  ⁢                                                                          ⁢                                      2                    n                                    ⁢                                                                          ⁢                  1                                                                                    -                                  C                                      c                    ⁢                                                                                  ⁢                    h                    ⁢                                          .2                      n                                        ⁢                                                                                  ⁢                    1                                                                                                          ⋮                                      ⋮                                                                          C                                                      c                    ⁢                                                                                  ⁢                    h                    ⁢                                                                                  ⁢                                          .2                      n                                        ⁢                                                                                  ⁢                                          2                      n                                                        -                  1                                                                                    C                                                      c                    ⁢                                                                                  ⁢                    h                    ⁢                                                                                  ⁢                                          2                      n                                        ⁢                                                                                  ⁢                                          2                      n                                                        -                  1                                                                                                        C                                                      c                    ⁢                                                                                  ⁢                    h                    ⁢                                                                                  ⁢                                          2                      n                                        ⁢                                          .2                      n                                                        -                  1                                                                                    -                                  C                                                            c                      ⁢                                                                                          ⁢                      h                      ⁢                                                                                          ⁢                                              2                        n                                            ⁢                                                                                          ⁢                                              2                        n                                                              -                    1                                                                                      ]            The leftmost value in each channelization code word corresponds to the chip transmitted first in time.
The channelization code, a spreading code, uses a different SF according to a rate of the user. The SF has a value of a multiple of 4, so that the SF has a value of SF=4 to SF=256.
As illustrated in FIG. 1, an increase in the SF value causes an increase in the number of available channelization codes. When a specific code is generated (divided) into two codes at each node, i.e., when the SF value is doubled, the generated two codes include one code calculated by doubling the specific code and another code calculated by adding the specific code to its inversed code. The UMTS system matches an SF value (=4 to 256) of the OVSF code to the data rate. For example, to maintain 3.84 Mchips/s, the UMTS system uses a low SF for a high data rate, and a high SF for a low data rate.
FIG. 2 shows how to spread and scramble a downlink physical channel. The downlink physical channel is subjected to complex multiplexing into I and Q channels, which are first multiplied by a channelization code for spreading and then, multiplied by a scrambling code for cell identification. FIG. 2 illustrates the spreading operation for all downlink physical channels except the Synchronization Channel (SCH), i.e. for P-CCPCH (Common Control Physical Channel), S-CCPCH, CPICH (Common Pilot Channel), AICH (Acquisition Channel), PICH, PDSCH (Physical Dedicated Shared Channel), and downlink DPCH (Dedicated Physical Channel). The non-spread physical channel consists of a sequence of real-valued symbols. For all channels except AICH, the symbols can take one of the three values of +1, −1, and 0, where 0 indicates discontinuous transmission (DTX).
Each pair of two consecutive symbols is first serial-to-parallel converted and mapped to an I and a Q branch. The mapping is such that even and odd numbered symbols are mapped to the I and Q branch, respectively. For all channels except AICH, symbol number zero is defined as the first symbol in each frame. For AICH, symbol number zero is defined as the first symbol in each access slot. The I and Q branches are then spread to the chip rate by the same real-valued channelization code Cch,SF,m. The sequences of real-valued chips on the I and Q branch are then treated as a single complex-valued sequence of chips.
This sequence of chips is scrambled (complex chip-wise multiplication) by a complex-valued scrambling code Sdl,n. In case of P-CCPCH, the scrambling code is applied aligned with the P-CCPCH frame boundary, i.e., the first complex chip of the spread P-CCPCH frame is multiplied with chip number zero of the scrambling code. In case of other downlink channels, the scrambling code is applied aligned with the scrambling code applied to the P-CCPCH. In this case, the scrambling code is thus not necessarily applied aligned with the frame boundary of the physical channel to be scrambled.
FIG. 3 illustrates how different downlink channels are combined. Each complex-valued spread channel corresponding to point S in FIG. 2, is separately weighted by a weight factor Gi. The complex-valued P-SCH and S-SCH are separately weighted by weight factors Gp and Gs. All downlink physical channels are then combined using complex addition.
The channelization codes of FIG. 2 are the same codes as used in the uplink, namely, OVSF codes that preserve the orthogonality between downlink channels of different rates and SFs. The channelization code for the Primary CPICH is fixed to Cch,256,0 and the channelization code for the Primary CCPCH is fixed to Cch,256,1. The channelization codes for all other physical channels are assigned by UTRAN. With the spreading factor 512, a specific restriction is applied. When the code word Cch,512,n, with n=0,2,4 . . . 510, is used in soft handover, then the code word Cch,512,n+1 is not allocated in the Node Bs where timing adjustment is to be used. Respectively, if Cch,512,n, with n=1,3,5 . . . 511 is used, then the code word Cch,512,n−1 is not allocated in the Node B where timing adjustment is to be used.
This restriction shall not apply for the softer handover operation or in case UTRAN is synchronised to such a level that timing adjustments in soft handover are not used with spreading factor 512. When compressed mode is implemented by reducing the spreading factor by 2, the OVSF code used for compressed frames is:Cch,SF/2,└n/2┘, if ordinary scrambling code is used.Cch,SF/2,n mod SF/2, if alternative scrambling code is used;where Cch,SF,n is the channelization code used for non-compressed frames.
In case the OVSF code on the PDSCH varies from frame to frame, the OVSF codes shall be allocated such a way that the OVSF code(s) below the smallest spreading factor will be from the branch of the code tree pointed by the smallest spreading factor used for the connection. This means that all the codes for UE for the PDSCH connection can be generated according to the OVSF code generation principle from smallest spreading factor code used by the UE on PDSCH. In case of mapping the DSCH to multiple parallel PDSCHs, the same rule applies, but all of the branches identified by the multiple codes corresponding to the smallest spreading factor may be used for higher spreading factor allocation.
The OVSF codes are structurally constructed in the form of a tree. In the light of the structural characteristic, if an upper code is used in the tree, it has no orthogonal property with every lower code, so that the lower codes cannot be used. If even one lower code is used, its upper codes cannot be used. For example, in FIG. 4, if an SF=4 code C4,0 is used, then every code in the tree of C4,0 including C8,0 and C8,1 is occupied, so that it is not possible to assign the lower codes. On the contrary, if a code C16,1 is used, not only C8,0 and C4,0, which are upper codes of the code C16,1, are occupied, but also lower codes of the code C16,1 are occupied.
This is merely a characteristic of the OVSF codes, not a problem. However, due to the characteristic of the OVSF codes, there occurs a problem. For example, there exist four SF=4 codes: one in each tree. Therefore, if a certain code in a lower part of the tree is assigned to a specific channel, it is not possible to assign new codes. In a sequential or random assignment method, there exists a previously assigned code in the middle of each tree during repeated assignment and release of the codes, so that in some cases, it is not possible to assign an upper code, i.e., a code for a higher data rate.
A need therefore exists for a method and system for downlink channelization code allocation or assignment in a UMTS which generate a code to be assigned to a new channel through a route having the minimum influence and which leave reserved codes by managing generation and release of the OVSF codes to prevent situations where the new channel cannot be assigned an OVSF code due to a lack of OVSF codes, even though there is enough available power and enough available channels.